Wireless electronic access and authorization is becoming more prevalent in many applications, including vehicles, homes, and office buildings. However, because such wireless methods are susceptible to fraudulent or unauthorized efforts, various techniques have been developed in response.
For example, passive keyless entry (“PKE”) is an automotive security system that operates automatically when the driver is in proximity to the vehicle, unlocking the door on approach or when the door handle is pulled, and locking it when the driver walks away or touches the car on exit. PKE systems are also used to secure buildings or areas of buildings.
A PKE device (sometimes generally referred to as a smart key) allows the driver to keep the key fob pocketed (or in a handbag) when unlocking, locking, and starting the vehicle. The key fob may be identified via one of several antennas in the car's bodywork and a radio pulse generator in the key fob housing. Depending on the system, the vehicle may be automatically unlocked when a button or sensor on the door handle or trunk release is pressed. Some vehicles with a smart key system allow the driver to activate the ignition without inserting a key in the ignition, provided the driver has the key fob inside the car. On such vehicles, this is commonly done by pressing a starter button or twisting an ignition switch. When leaving a vehicle that is equipped with a smart key system, the vehicle may be locked by either pressing a button on a door handle, touching a capacitive area on a door handle, or simply walking away from the vehicle. Some vehicles automatically adjust user customized settings based on the particular smart key used to unlock the car. User preferences such as seat positions, steering wheel position, exterior mirror settings, climate control (e.g., temperature) settings, and stereo presets are popular adjustments. Some vehicle models even have settings to prevent the vehicle from exceeding a maximum speed if it has been started with a certain key fob.
Communications between a PKE device and the vehicle are performed using wireless radio frequency (“RF”) systems, and more particularly wideband RF applications such as Ultra-Wideband (“UWB”) technology, which are capable of accurate distance measurements between two or more wireless devices. Typically, these measurements are based on Time-of-Flight (“ToF”) calculations that are derived by accurate determinations of departure and arrival times of RF packets between the two devices. The high bandwidth of UWB equates to short pulses, which means that high ranging precision in short measurement times can be achieved with this technology. RF packets travel at the speed of light, and thus a calculated ToF allows a determination of the distance between the devices. Such a procedure is commonly referred to as “ranging” (also referred to herein as a “ranging operation”). One practical application of ranging is “distance bounding” whereby ToF calculations are used to verify whether the distance between two devices is less than a predefined threshold. Ranging and distance bounding can be used for PKE systems and other access control systems, as well as for contactless electronic payment systems.
A drawback of the high bandwidth of UWB is that a receiving device should employ a very wide channel filter and process the data at a very high sampling rate. A consequence of these design constraints is a high current consumption of the receiving device. This is especially problematic for small battery powered devices. A coin cell battery may only be able to deliver <10 mA at low temperatures, especially an older coin cell battery. The receiving device, however, may require >100 mA peak current in order to satisfactorily receive and process the UWB signals. Therefore, an energy storage capacitor is typically required to bridge the gap between the current the receiving device requires and the current the battery can deliver. This is especially problematic for PKE systems in automotive applications since a larger energy storage capacitor requires a longer time to recharge after discharge, which slows down the ability of the receiving device to process successive incoming signals from the vehicle. It can be readily appreciated that a vehicle designer/engineer does not want an electronic system that experiences an unsatisfactory delay for a car door to unlock or an ignition to fire.